The invention relates to an apparatus for projecting a light beam onto an optically readable record carrier, which apparatus comprises an optical detector having at least two subdetectors for detecting the light beam which has been modulated by the record carrier,
a focussing device for focussing the light beam on the record carrier,
a circuit for generating a focussing error signal which comprises a first signal corresponding to the difference between the two signals supplied by the subdetectors, and
a focussing correction device for adjusting the focussing device in response to the focussing error signal.
When such an apparatus is switched on it is common practice to disconnect the focussing error signal from the focussing device by means of a switch and to apply an increasing signal to the focussing correction device in order to move the focussing correction device towards the record carrier. In the meantime the focussing error signal is monitored to detect whether the desired control point is approached within a specific range and subsequently apply the focussing error signal again, after which the control loop is closed and the focussing is controlled by means of the focussing error signal.
In order to obtain a correct servo control the response of the focussing error signal near the in-focus point should be high. However, this results in oscillations about the in-focus point when the known lock-in method is used.